The present invention relates to switching regulator circuits. More particularly, the present invention relates to circuits and methods for controlling the frequency of switching voltage regulators using voltage controlled oscillators.
The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a voltage source which may be poorly-specified or fluctuating. A typical switching regulator employs a switch including a switching element (e.g., a power transistor) coupled either in series or parallel with the load. The switching regulator controls the timing of the turning ON and turning OFF of the switching element in order to regulate the flow of power to the load. Typical switching regulators employ inductive energy storage elements to convert switched current pulses into a steady load current. Thus, power in a switching regulator is transmitted across the switch in discrete current pulses (versus a linear regulator where power is transmitted across the pass element as a steady flow of current).
In order to generate a stream of current pulses, switching regulators typically include control circuitry to turn the switch ON and OFF. The switch duty cycle, which is generally defined as the percentage of time that the switch is ON relative to the total period of the oscillator, can be varied by a variety of methods. In many instances, the switching regulator needs to be synchronized to an external source. This may be accomplished by injecting an external SYNC signal into the oscillator circuit to cause the oscillator to match the SYNC signal (as is well known, the SYNC signal can only be used to speed up the oscillator circuit). In commonly used sawtooth oscillators, the voltage on a capacitor ramps up to a threshold voltage at which time the voltage resets and the cycle repeats. The effect of the SYNC signal is that the oscillator ramp signal is effectively cut short and restarted so that the oscillator produces more cycles in a given time period, but the oscillator ramp signal has a lower amplitude (because each oscillator cycle has less time to ramp up).
While pre-termination of the oscillator may not be a problem in many switching regulators, pre- termination tends to cause problems in current-mode switching regulators (i.e., switching regulators that are controlled by a current signal). One of the reasons for these problems is the fact that current-mode regulators tend to become unstable when the duty cycle exceeds 50% (i.e., when the switch is ON for more than 50% of a given oscillator period). Stability is often maintained in current-mode regulators above 50% duty cycle by injecting a slope compensation signal into the current signal. One method of producing the slope compensation signal is to "pick off" a portion of the oscillator signal to be the compensation signal. This portion, however, may be unavailable when the oscillator ramp signal is pre-terminated. Under those circumstances, the slope compensation signal is often insufficient to maintain stability in the regulator.
An additional deficiency with known switching regulators that are synchronized to a given frequency is related to the production of noise. When a regulator is held at a single frequency, such as 100 khz, noise is produced at that frequency (e.g., from the switch drivers, etc.) and at each of the harmonics of that frequency. Although the noise may be irrelevant in many circumstances, there are many other circumstances where the noise tends to affect other (i.e., non-regulator) circuit components. The noise produced at the fixed frequency tends to have a signal that is very narrow and has a relatively high amplitude (i.e., focused at the frequency of the regulator and its harmonics).
In view of the foregoing, it would be desirable to provide switching regulator circuits that include the capability to be synchronized to external signals without interfering with the slope compensation circuit.
It would also be desirable to provide switching regulator circuits and methods for synchronizing the regulators while insuring that the oscillator ramp signal reaches its predetermined peak amplitude.
It would be further desirable to provide circuits and methods for reducing effects of the noise produced in switching regulators.